1. Field of the Invention
The present invention relates to a paper feed apparatus employed in printers, copying machines and the like, and more particularly to a paper feed apparatus for feeding paper, one sheet after another, by means of a feed roller from a cassette containing sheets of paper stacked one upon another.
2. Description of the Prior Art
In a paper feed apparatus of the type which has a cassette mounted therein that contains a stack of papers and which is adapted to feed the papers one by one by means of a feed roller, means for separating one sheet from another is conventionally in the form of separation pawls provided at corners of the leading end portion of the cassette as seen in the direction of paper feed (said leading end portion to be hereinafter referred to as "forward end").
Such arrangement is effective when papers having a predetermined size are used. If the size of the papers is changed, however, the separation pawls will not work effectively.
With a view to eliminating such drawback, the present inventor, in his Japanese patent application No. 24432 of 1983, disclosed a paper feed apparatus for feeding paper, sheet by sheet and sequentially from the uppermost sheet, by means of a feed roller from a stack of papers contained is a cassette, characterized by upwardly extending pawls bent forward in two stages at the upper end portion thereof and provided at the forward end of the cassette, and a pressure plate bent upwardly at the front end portion thereof and pivotally movable about the rear end thereof, said pressure plate being mounted in the upper front portion of the cassette so that its front end is positioned adjacent said pawls.
Such paper feed apparatus is ellustrated in FIGS. 1 to 4, inclusive. Referring to FIGS. 1 and 2, there is shown a cassette employed in the paper feed apparatus, the body of the cassette being designated by reference numeral 1. Numeral 2 is a lifting plate fitted into position on the bottom of the cassette body 1. The lifting plate 2 has projections 2a, 2b at the rear end thereof as seen in the direction of paper feed (hereinafter referred to as "rear end"), said projections 2a, 2b being loosely fitted into corresponding slots provided in the bottom of the cassette body 1 at the rear end thereof so that the lifting plate 2 is pivotable about the projections 2a, 2b. The lifting plate 2 also has a notch 3 provided at a location corresponding to that of a guide plate 8 which will hereinafter be described. Shown at 4 are separation pawls disposed upright at the inner side of the forward end of the cassette body 1. The upper end portion of each separation pawl 4 comprises an oblique portion 4a bent upward at a certain angle to a verticle plane, and a vertical portion 4b extending upright from the upward end of the oblique portion 4a. Both separation pawls are disposed so that the midpoint of the space between them aligns with the widthwise center of a feed roller 5 fixed to the body of the apparatus in which the cassette is mounted, and so that said midpoint is substantially laterally displaced from the center of the forward end of the cassette body 1, to the left as seen from the front side. It is noted that the term "left" (or "right"), whenever used hereinafter, refers to the left (or right) as viewed from the front side of the cassette body 1. Numeral 6 designates a pressure plate having projections 6a, 6b extending downward at both sides of its rear end and pivotally supported through said projections on side plates 1L, 1R of the cassette body 1 at respective upper portions thereof. The forward end portion of the pressure plate 6 that is not in opposed relation to the feed roller 5 is positioned adjacent the separation pawls 4, and plate 6 is spaced gradually further from the bottom of the cassette body 1 in a direction of extension away from the separation pawls 4, so that the pressure plate 6, at said forward end portion thereof, is allowed to press the papers most positively at portions thereof adjacent the separation pawls 4. The forward end portion of the pressure plate 6 includes upward bent portions 6' and a notch 7 framed at a position at which the forward end portion is opposed to the feed roller 5. Designated by numeral 8 is a guide plate for guiding stacked papers in position in the cassette body 1 and along the right side plate 1R thereof, said guide plate 8 being disposed at a location corresponding to that of the notch 3 provided in the lifting plate 2. The guide plate 8 has an L-shaped channel configuration with its horizontal portion pinned in a slit-like elongated hole 9 formed in the bottom of the cassette body 1 so that the plate 8 is slidable in the widthwise direction of the stack of papers in the cassette body 1. By changing the position of the guide plate 8 is it possible to adapt the cassette body 1 for housing therein various different sizes of paper as required.
The mode of operation of the paper feed apparatus employing the aforesaid cassette will be explained with reference to FIGS. 3 and 4.
A stack of paper 10 placed on the lifting plate 2 and, guided in position by the guide plate 8, is lifted along with the forward end of the lifting plate 2 by a lifting lever 11 which is provided in the body of the apparatus (in which the cassette body 1 is mounted) and which is extensible into the cassette body 1 under the pressing force of spring means (not shown), the uppermost one of the papers being contacted with the feed roller 5. When the feed roller 5 is rotated clockwise as viewed in FIG. 3, the uppermost sheet 12 in the stack of papers 10 is advanced, its front end being first brought into contact with the oblique portions 4a and then with the vertical portions 4b of the separation pawls. Meanwhile, the surface of the sheet 12 is pressed by the bent portions 6' of pressure plate 6.
With such arrangement, however, there is a difficulty that since the front end of the sheet 12 is caused to advance from the oblique portions 4a to the vertical portions 4b of the separation pawls while being held in contact therewith and since the surface of the sheet 12 is pressed by the bent portions 6' of the pressure plate 6, a feed error is likely to occur, if a relatively large frictional force is involved between the sheet 12 and the separation pawls 4, and more particularly between the sheet 12 and the bent portions 6' of the pressure plate 6. That is, the frictional force due to the pressure applied by the bent portions 6' of the pressure plate 6 on the sheet 12 may be greater than the force exerted by the feed roller 5 for moving the sheet 12 forward, and accordingly the sheet 12 may be prevented from being fed foward.
Another difficulty is that since the uppermost sheet 12 of the sheets contained in the cassette body 1 is supported, at its right edge side, only partially by the guide plate 8, it may be fed in an oblique posture with its rear end inclined rightward, if its condition of contact with the feed roller 5 is unbalanced about the center thereof, for example, if its portion adjacent the right side plate 1R is moved faster than the other portion.
A further difficulty is that, as the papers in the cassette run short, two or more sheets of paper are likely to be fed at a time. That is, when the stack of papers 10 is thinned to the extent that only a few sheets are left, the lifting plate 2 is raised and the lifting force of the lifting lever 11 is weakened. Consequently, the frictional force between the lifting plate 2 and the lowermost sheet in contact therewith becomes smaller than the frictional force between the lowermost sheet and the immediate next upper sheet or sheets, and thus all the remaining sheets of the stack of papers 10 are fed at the same time. Such phenomenon is most likely to occur when two or three sheets of paper are left of a stack of papes 10.